The following information and free downloads will provide assistance when using Adobe Illustrator or Photoshop in designing and saving files for two color printing.
How do I design for 2 color printing with Illustrator?
For instance, can two colors be combined for a third color? Can the white of the paper be allowed to shine through to create gradients (like light blue to dark blue)? Does Illustrator allow you to select just two colors and design from those. If so how would you set up your color palette?
Sure, you can use Illustrator to design your pieces with only two colors. You'll be working with Spot Colors.
Working in a CMYK document color mode, you can create your CMYK color and add it to the Swatch Window (Window > Show Swatches).
Then you can make it a spot color by double clicking on it in the Swatch Window. Change the Color Type from Process to Spot.
Notice the 'spot' in their little swatch in the Swatch Window. Or there are entire libraries available from Pantone, who supplies them to Adobe. Windows>Swatch Libraries>Pantone (Coated, uncoated, etc, depending on the gloss of your paper stock)
To vary the screen from 0 to 100 percent, use a percentage tint (Window > Show transparency)
To blend two colors, you need to place objects on top of each other and learn about Overprinting. For example. Create a 100% red spot color square and a 100% yellow spot color square and overlap parts of them so there's a common area.
Now select View > Overprint Preview from the menu
Now select both squares and choose Window > Show Attributes. There, check the box marked Overprint Fill.
The common area will become orange and that's the way it will print!
That's how you create interesting blends with a limited spot color palette.
If one of your colors is black, you can just use 100%K, you don't need to Spotify it.
Please be aware that there are certain issues with printing pieces that use transparency. For more information here's a guide from Adobe about Flattening your Transparencies before printing.
Printing in two colors using a grayscale image or graphic and spot color can be an effective and inexpensive alternative to process color printing. The following link is a guide from Adobe Photoshop to prepare full-color images for two-color printing.
Two-Color Printing with Photoshop